In many of conventional image forming apparatuses of electrophotography mode, such as a photocopier, a printer or a facsimile, a toner image is first formed on a photoconductor, and the toner image is then transferred to a recording paper by application of electric field. After that, a fixing apparatus heats up the toner image and the recording paper, so that the toner image is fixed to the paper permanently. The fixing apparatus used in such an image forming apparatus is generally constituted of a heating roller and a pressure roller, and a recording paper on which the toner image has been transferred is fed between a fixing nip section provided between two rollers, so that the toner image is fixed onto the recording paper by heat and pressure.
Another well-known fixing mode of image forming apparatus is so-called a simultaneous transferring and fixing mode. In this method, the toner image is formed on the surface of a photoconductor, and then is transferred to an intermediate transfer member. The toner image is heated on the intermediate transfer member, and then is transferred to a recording paper and is fixed thereto at the same time.
The simultaneous transferring and fixing mode is disclosed, for example, in Document 1 (Japanese Unexamined Patent Publication Tokukai 2000-194205 (published on Jul. 14, 2000)). As described therein, this mode allows transfer of toner image onto a recording paper by a non-electrostatic manner (such as heat), and therefore hardly causes degradation of image due to disturbance of electric field. With this advantage this mode ensures high picture quality.
The fixing apparatus of simultaneous transferring and fixing mode disclosed in Document 1, that is, the transferring and fixing apparatus is structured as follows. The transferring and fixing member includes an intermediate layer, which is not less than 0.25 mm, more preferably not less than 1 mm in thickness, on its base, and an outermost layer with a small surface energy is provided on the intermediate layer. The outermost plane of the member is covered by silicone rubber, fluorocarbon rubber, or fluorocarbon polymer. The transferring and fixing member is constituted of a belt or a roller-structure body. The pressure member pressed into the transferring and fixing member is constituted of a roller-structure body. In the case where the transferring and fixing member is a belt-structure body, a roller containing a heat source is provided to be opposed to the pressure roller, and a fixing nip section is formed between the transferring and fixing member and the pressure roller.
The conventional fixing mode which carries out fixing and transfer separately is almost immune to a stain on the surface of fixing member, or partial loss of image, which occurs when the toner image formed on the recording paper is transferred onto the surface of the fixing member in the fixing process. This defect is easily prevented if the detachment performance of the surface of fixing member with respect to the toner used is fully ensured, and the temperature is optimally set.
On the other hand, in the simultaneous transferring and fixing mode, it is necessary to ensure complete transfer of the toner image formed on the transferring and fixing member onto the recording paper in the transfer fixing nip section. If the toner image is somehow not to be completely transferred onto the recording paper and some toner is left on the transferring and fixing member, some kind of image defect occurs. Further, the residue toner may be brought into contact with the recording paper again by the rotation of the transferring and fixing member, which makes a stain on the paper.
In the simultaneous transferring and fixing mode, the transfer of toner from the transferring and fixing member to the recording paper is carried out by heat. Therefore, to solve the foregoing problem, it is necessary to ensure adhesion between the recording paper and the surface of the transferring and fixing member to enable the toner to be completely transferred from the transferring and fixing member to the recording paper.
For example, Document 1 improves the adhesion between the transferring and fixing member and the recording paper (the subservience of the surface of the transferring and fixing member with respect to the recording paper) in the following manner. The thickness of intermediate elastic layer formed on the base of the transferring and fixing member has a thickness of 1.0 mm or greater. To ensure sufficient detachment performance of toner from the transferring and fixing member, it is necessary to coat the outermost plane with fluorocarbon resin or the like. However, the coated surface of the transferring and fixing member is hard, and the adhesion with the recording paper greatly decreases.
Further, as described, in the simultaneous transferring and fixing mode, the toner is transferred onto the recording paper by heat, and therefore the transfer performance especially for a dot image decreases, and thereby various defects such as degradation of image is more likely to occur.